This is a study to evaluate the safety and find the highest tolerable dose of Levulan® and Photodynamic Therapy (PDT) for premalignant tumors of the head and neck. Subjects will receive Levulan® by mouth and will undergo PDT. Levulan® goes throughout the body, but gets retained in cancer cells more than non-cancer cells. After 4-6 hours, the targeted area is treated with laser light which activates the Levulan® to kill the cells that contain it.